This research will explore the acoustic dimensions of auditory perception in anuran amphibians to determine the limits of the acoustical world of these animals. Experiments will identify the acoustic features which influence the detection and discrimination of biologically relevant sounds from other sounds in the environment. A powerful new technique which measures the performance of the anuran auditory system to its real limits will be employed to test the prevailing "matched filter" hypothesis of vocal communication in anurans and to take advantage of these unique animals to develop our understanding of the comparative aspects of hearing in vertebrates. This new technique, reflex modification, is the first sensitive psychophysical procedure yielding precise, quantitative data on auditory detection in anurans. The results of the proposed experiments will be relevant to hypotheses concerning specializations of the auditory system for detecting sounds of biological interest to animals, and will also contribute to the understanding of the relation of morphological structures in the inner ear to hearing capabilities in vertebrates. This research can provide important contributions to the scientific disciplines of sensory physiology and communication sciences. The development of a sensitive procedure for audiometric measurements in anurans based on reflex modification has implications for the study of sensory function in other animals for which alternative measures based on standard conditioning techniques are inefficient or difficult to achieve. Because reflex modifiation requires no training and produces effects with relatively few trials, it may be a useful procedure for clinical and diagnostic work with infants and children, and for assessing effects of various drugs on sensory function.